How do many different birds live in one place?

By BILL CHAISSON
Of a Feather
“Five species of warbler, Cape May, myrtle , black-throated green, blackburnian, and bay-breasted, are sometimes found together in the breeding season in relatively homogeneous mature boreal forests. These species are congeneric, have roughly similar sizes and shapes, and all are mainly insectivorous. They are so similar in general ecological preference, at least during years of abundant food supply, that ecologists studying them have concluded that any differences in the species’ requirements must be quite obscure.”

So begins a pivotal 1958 paper in the journal Ecology written by Robert H. MacArthur of the University of Pennsylvania. After two seasons of field work in the Maine woods, MacArthur discovered how these warbler species managed to partition an apparently homogeneous resource and coexist, yea thrive, together. The paper, “Population Ecology of Some Warblers of Northeastern Coniferous Forests,” revolutionized population and community ecology with its combination of insight and logical rigor. Up to this time ecology had tended to be primarily observational, but MacArthur’s approach was both observational and mathematical.

MacArthur grew up in Marlboro, Vermont and attended Marlboro College, where his father was a professor. He went to Yale to do his Ph.D. and was a student of G. Evelyn Hutchinson, an early pioneer in bringing a mathematical approach to ecology.

The seminal warbler paper was an extract of his doctoral dissertation. It includes five illustrations of a spruce tree that are a model of reductionism. A tree is represented as an isosceles triangle divided horizontally into five layers plus the sub-canopy layer to the forest floor. Most of the trees were 50 to 60 feet tall, so his zones were roughly 10 feet high. MacArthur then divided the tree vertically with two parabolic lines descending from the peak to two points in the lowest layer of the canopy. These divided the branches into an outer “terminal zone” (the most recent year and a half’s growth), a middle zone of older needles, and the inner bare or lichen-covered portion of the branches nearest the trunk.

Using the representative tree divided into 16 foraging regions (3 in each of the five layers of the canopy and one beneath it), MacArthur tallied his observations of the five warbler species. He counted the number of observations per species in a given zone and the length of each observation in seconds. The illustrations in the paper show the tree split down the middle, with the number of observations on the right and the number of seconds of observation on the left, both expressed as percentages. He added up the areas of highest percentage until they exceeded 50 percent and showed these areas as stippled.

The Cape May warbler, he showed, feeds almost exclusively in the terminal branches of the top 20 feet of the spruce. The myrtle warbler feeds primarily on the inner bare branches of the lowest layer of the canopy and beneath it. The black-throated green warbler was observed mostly in the middle two layers in the terminal and middle regions of the branches. The blackburnian warbler spent most of its time in the terminal regions of the top three layers, but it was observed most often in the middle region of the branches in the top two layers. The bay-breasted warbler concentrated its activity in the middle layer of the tree and ranged from the terminal region right into the trunk.

Even this initial computation of “time spent” showed that each warbler species did the bulk of its foraging in a different part of the tree. But there was overlap. “Early in the investigation,” wrote MacArthur in his habitually lucid way, “it became apparent that there were differences between the species’ feeding habits other than those of feeding zones. Subjectively, the black-throated green appeared ‘nervous,’ the bay-breasted slow and ‘deliberate.’ In an attempt to make these observations objective, the following measurements were taken on feeding birds. When a bird landed after a flight, a count of seconds was begun and continued until the bird was lost from sight. The total number of flights (visible uses of the wing) during this period was recorded so that the mean interval between uses of the wing could be computed.”

It was just this kind of objective and quantitative approach that distinguished MacArthur’s work from that of many of his contemporaries. Furthermore, he followed up the gathering of data with sophisticated analysis grounded in statistics and calculus. By means of a sign test he determined that the black-throated green was the most fidgety, the blackburnian and myrtle were less so, and the Cape May and bay-breasted were the most slow and deliberate feeders.

Unsatisfied with this measurement he quantified each species’ spatial movements along three axes – vertical, radial and tangential – after noticing that some species tended to hop from place to place while foraging while others tended to “crawl.” He found that the Cape May moves predominantly in a vertical direction, the black-throated green and myrtle in a tangential direction and the bay-breasted and the blackburnian in a radial direction. This was represented in an equilateral triangle with dots at the intersection of three axes that extended from each of the three sides. The length of the each axes representing the number of times each species was observed to move predominantly in a particular direction.

MacArthur measured other aspects of each warbler species’s behavior – including height of the nest above the ground – and defined their niches using several factors, all presented numerically and shown to be significant (i.e. real) via statistics. It was a stunning illustration of the principle of competitive exclusion on a meso-scale. He showed that by close observation and careful measurement you may characterize how species partition resources in order to coexist in an environment that looked “all the same” by casual inspection.

Bill Chaisson, who has been a birdwatcher since age 11, is a former editor of the Eagle Times. He now works for the Town of Wilmot and lives in Sutton.